Photo by Vicky DeLoach / (CC BY-NC-ND 2.0)

Light pollution keeps them awake

Sleepy geese

Photo by Vicky DeLoach / (CC BY-NC-ND 2.0)
Just like people, geese get less sleep when it’s light outside. But geese also sleep less during cloudy nights. Sjoerd van Hasselt wondered why. The answer? Light pollution.
By Loes van de Straat
18 October om 13:50 uur.
Laatst gewijzigd op 20 October 2021
om 11:42 uur.
October 18 at 13:50 PM.
Last modified on October 20, 2021
at 11:42 AM.

Geese researcher Sjoerd van Hasselt starts smiling widely the moment he walks into the animal testing facility at the Linnaeusborg and sees all the cages and aviaries. He can hear the loud singing and twittering coming from the starling aviary, which means he’s nearing his goal: the much quieter geese.

There are thirteen barnacle geese, divided across two large aviaries. It’s not exactly the Hilton hotel: there’s food and water, and one side of the aviary had fresh air and light. More interestingly, though, each goose has a little gizmo on its head.

Lego brick

It almost looks a little silly, as though someone glued grey Lego bricks to their heads, but it doesn’t appear the animals are troubled by it in any way. ‘They’re not’, Van Hasselt says. ‘The geese are big enough that the device doesn’t bother them.’

That being said, the geese did have to undergo a small surgery to install the data logger. Five small holes were drilled into their skulls to insert electrodes, which measure the animals’ brain activity. ‘But they recover from the surgery perfectly well’, Van Hasselt explains. ‘With no discernible behavioural changes.’ That, he knows, means the device doesn’t bother them. 

There’s a big lack of knowledge when it comes to sleep

That’s a good thing, because the devices provide Van Hasselt with data on when the birds are awake and when they’re asleep. It’s information that’s sorely needed. ‘We know very little about how sleep works in birds’, says Van Hasselt. 

REM sleep

‘We know that mammals need a lot of REM sleep and a lot of non-REM sleep’, says Van Hasselt. REM means rapid eye movement, which is what happens during this stage of sleep. But the need for REM sleep varies quite a lot in birds, Van Hasselt says. ‘It ranges from almost no need for REM to a need that’s equal to that of mammals.’

But why is this? And what does it mean? ‘What we know about sleep changes from species to species’, says Van Hasselt. ‘There’s a big lack of knowledge when it comes to sleep.’ 

But the post-doctoral researcher has already made a surprising discovery: When he analysed his data, he saw that geese slept less during cloudy nights. And that’s weird.

Full moon

‘We already knew that geese sleep less during a full moon’, Van Hasselt says. ‘There’s more light during those nights than during a new moon, for example.’ The extra light means they can see their enemies better, and they use the advantage to eat more.

Van Hasselt expected the geese to sleep more during cloudy nights because, he reasoned, it would be darker. ‘Except it turned out I was wrong. The cloud cover reflects the artificial light from streetlights or the letters on the Linnaeusborg, which means there’s just as much light at night as during a full moon’, he explains.

We don’t know if the birds will suffer any long-term negative effects 

Is this a bad thing? Van Hasselt doesn’t really know. The light means the geese can eat more, and more food means more energy. It doesn’t appear to have any negative short-term effects.


However, Van Hasselt worries it might affect them on the long term. He knows that people who miss out on sleep over a longer period of time suffer ill effects, both physically and mentally. ‘Light pollution is unnatural. We should have as little of it as possible.’ 

‘But we don’t know nearly as much about birds. We simply don’t know anything about the long-term negative effects of artificial light on the health of birds.’

Van Hasselt has since moved on to a new topic, although it’s still in the realm of geese and their sleeping patterns. He’s currently trying to find out more about how the moon influences his geese’s sleep.

Magnetic field

Geese are probably able to perceive the earth’s magnetic field, which they use to navigate during their spring and fall migrations. But the moon can disrupt this magnetic field, and this could mean that geese adjust their sleeping to this.

I had to climb all the way on top of the aviaries sometimes. It was pretty tricky

‘There’s a lot of light pollution in our current society. I want to know if the geese need to only see the moon or other light sources just to change their behaviour’, says Van Hasselt. ‘Or perhaps there’s more going on, and they also need to perceive changes in the earth’s magnetic field before making any changes?’

In order to answer that question, he covered the geese cages with large pieces of plastic during the full moon and on the new moon. He used black plastic to block out the moon and make it dark, and see-through plastic to let the light in. ‘I had to climb all the way on top of the aviaries sometimes’, he says, laughing. ‘It was pretty tricky.’


Right now, Van Hasselt is working on processing the results of this study. He already has inkling about the results, though. While the geese slept less at night during a full moon in winter, they actually slept less during the day in summer. The fact that they sleep less during a full moon therefore can’t be due to the moonlight alone. 

Since geese are also able to perceive the earth’s magnetic field, Van Hassel thinks this also plays an important role in the geese’s sleeping patterns. Now he just has to wait and see if the results confirm this.